This invention relates to ship propulsion systems, and more particularly to a control for a propulsion system which includes a simplified control of a propeller shaft brake.
A common form of marine propulsion system uses ahead and astern air actuated clutches for connecting the prime mover to a reversing reduction gear unit which drives the propeller. The air actuated clutches are engaged by inflation and the degree of clutch engagement can be controlled by controlling the amount of inflation. A pneumatic control system is typically provided for selecting the ahead or astern clutches, for controlling the clutch inflation, and for controlling an engine speed governor which determines the engine speed. A single throttle lever apparatus can be provided for controlling both the clutch engagement and engine speed by movement of the lever from neutral towards either an ahead or astern direction. Examples of such a control for a ship's propulsion system are found in John M. Phinney U.S. Pat. No. 3,727,737, issued Apr. 17, 1973 for "Pressure Modulating System for Reversing Clutches and Throttle Control" and U.S. Pat. No. 4,072,221 issued Feb. 7, 1978 for "Marine Clutch and Throttle Governor Control System."
In the systems of these earlier patents, a pneumatic clutch control assembly for a ship's propulsion system is sequentially operated to regulate the inflation of ahead and astern air inflatable clutches and to also control the prime mover speed. The control assembly is actuated by a single throttle lever located on a pilot house control stand. Movement of the lever in one direction provides forward rotation of a propeller at a speed which increases with handle travel away from neutral. Movement of the handle in the opposite direction provides astern rotation of the propeller with speed increasing as the handle is moved farther from neutral. The center position provides a neutral setting in which the engine is disconnected from the propeller and no power is transmitted, although the engine continues to idle.
The single lever control of both direction and speed is accomplished in the following manner: As the lever is pivoted in either direction from neutral, air is supplied to a selector valve which selects one or the other of the ahead or astern clutches. Thereafter, and up to a first control pressure, air pressure proportional to the position of the lever away from neutral is fed through a first valve to the selected clutch and begins inflating the selected clutch. During this time the engine remains at idle speed. After a first control pressure is reached, the first valve is piloted and a second path for air to the clutch is established. This second path has provision for an initial programmed rate of feed of air to the clutch through a choke valve so as to softly inflate the clutch. Upon reaching a second higher control pressure, full supply air pressure is connected to the clutch. After the first control pressure is reached, the continued inflation of the clutch is not dependent upon the position of the throttle lever.
When the air pressure within the clutch rises to a predetermined level, the control of these earlier patents pilots a governor valve which, in effect, connects the throttle lever control to the speed governor of the engine so that the pressure supplied to the governor directly corresponds to the position of the throttle lever and the engine speed is controlled by movement of the throttle lever. The throttle lever setting determines only the final operating speed and direction. All intermediate steps of clutch engagement and inflation, as well as engine governor speed, are handled automatically by the control system.
Certain conditions of operation of ship propulsion systems require sudden reversals of direction. A typical situation in which sudden reversal is required is the so-called "crash reversal" in which the ship's pilot commands a sudden reversal of the direction of rotation of the propeller from a medium or high forward speed to reverse direction in order to prevent a collision. In such situations, the clutches are subjected to considerable load because of the need to first halt the rotation of the propeller shaft in one direction before it can be started to rotate in the opposite direction. This can result in the build-up of considerable heat in the clutches which, if severe enough, can lead to failure of the clutches. To protect the clutches in such situations, it has been typical to apply a brake to the propeller shaft when a shift is commanded from high speed travel in one direction to the opposite direction. The purpose of the brake is to absorb heat and relieve the clutches of a portion of the heat load which would otherwise be imposed upon them during high speed maneuvers.
When propeller shaft brakes have been used in the past, the control systems have functioned to apply the brake during a delay period in neutral when neither the ahead nor astern clutches are engaged. Past systems have also typically applied the brake without regard to whether the reversal in direction was from ahead to astern or astern to ahead. Examples of such systems are found in to Harold M. Mathers U.S. Pat. No. 3,543,891 issued Dec. 1, 1970 for "Controls for Engine, Brake and Forward/Reverse Clutches" and U.S. Pat. No. 3,669,234 issued June 30, 1972 for "Fluid Controls for Engine and Forward/Reverse Transmission." Another example of such a system is found in U.S. Pat. No. 3,900,090 issued Aug. 19, 1975 to Jack R. Kobelt for "Engine Remote Control." A disadvantage to the use of a delay period during which the brake is engaged is that it lengthens the time required to alter the ship's direction and it creates a period during which the ship is not under full control. Further, astern direction travel is most often carried out at a low speed so that shifting from astern to ahead rarely overloads the clutches, and the time delay in engagement of ahead direction clutches is unnecessary when a reversal from astern to ahead is commanded.
Another approach involving the use of a maneuvering brake is found in U.S. Pat. No. 4,119,185 issued Oct. 10, 1979 to John M. Phinney for "Marine Propulsion Control System with Maneuvering Brake." That system operates only when a reversal from medium or high speed ahead operation is commanded. However, it too introduces a delay in the inflation of the astern clutch, which delay is dependent upon the forward speed.
I have provided a simplified control for a maneuvering brake which is effective in its operation but does not require a delay in the engagement of the astern clutch when reversal from medium or high forward speed is commanded.